1. Field of the Invention
The present invention relates to an apparatus for forming an image with use of electrophotographic process, such as a digital copying machine, a digital printer or the like.
2. Description of Related Art
Various kinds of an apparatus such as a laser printer have been used practically for forming an image with use of electrophotographic process by exposing the photoconductor with a light beam according to image data which have been converted into digital values, while various kinds of methods for forming a digital image have been suggested in order to reproduce a half-tone image such as a picture with high fidelity.
Among such methods are known an area gradation method with use of a dither and matrix a multi-level laser exposure method (such as the pulse width modulation method or the light-intensity modulation method) wherein the gradation for a dot to be printed is expressed by the quantity of laser light (which is equal to the product of the light-emitting time and the light intensity) by changing the pulse width (light-emitting time) or the light intensity, (refer for example Japanese patent laid-open publication No. 91077/1987), No. 39972/1987, No. 188562/1987 and No. 22597/1986). Further, multi-level dither methods in combination of the dither method with the pulse width modulation method or light intensity modulation method are also known.
It is possible in principle to reproduce an image density with gradation by using one-to-one corresponce uniquely to the gradation of image data to be reproduced by using such a method. However, the image density to be reproduced (hereinafter referred to as image density) is not proportional to the density of a document owing to complex factors such as the photosensitive characteristic of the photoconductor, the characteristics of toners and the environment to be used. The nonlinear characteristic which deviates from the linear characteristic to be realized is called the gamma characteristic generally or the gradation characteristic, and this nonlinear gamma characteristic is one of the important factors to lower the fidelity of a reproduced image especially of a half-tone document.
In order to improve the fidelity of a reproduced image, the so-called gamma correction is adopted so that an image can be reproduced with fidelity according to the document image. In the gamma correction, a document density data to be read is transformed by using a predetermined transformation table for gamma correction, and a digital image is formed according to the transformed document density.
Further, there are many factors which affect the image density. One of the factors is a phenomenon that the amount of toners adhered to the photoconductor on the development varies with the changes in external environment such as temperature or humidity according to the characteristics of the photoconductors and the toners. It is known generally that at an environment of high temperature and high humidity the amount of the adhered toners increases and the gamma characteristic increases sharply around the origin, so as to make the density of the reproduced image large, whereas at the environment of low temperature and low humidity the amount of the adhered toners decreases and the gamma characteristic increases mildly around the origin, so as to make the density of the reproduced image small.
In order to stabilize the image density against such environmental change, the toner density is controlled so as to keep the maximum image density constant in a copying machine or a printer with use of electrophotographic process.
In a method adopted generally for the toner density control, the surface electric potential of the photoconductor and the development bias electric voltage are changed manually or automatically according to the toner density of a standard toner image formed on the photoconductor.
However, such changes in surface electric potential and development bias electric voltage affect the gamma characteristic largely.